How to properly remove pointers of objects from a dynamic memory?

I have a method that when given a command, removed all pointers of objects stored in a vector. These pointers were all created on the heap using new command. This is what I have for the method and I am getting segmentation fault 11 error.

else if (command == "Clear") {
    size_t allGrades = grades.size();
    for (size_t i = 0; i < allGrades; ++i) {
        grades[0] = nullptr;
        delete grades[0];
    }
}

where grades is the vector of pointers that have been allocated on the heap. How would I fix this?